In general, power supply circuits are used to couple a primary power source to a load. Typically, power supply circuits modify the electrical characteristics of the primary power source to provide power having electrical characteristics that are compatible with the load. For this reason, power supplies are often referred to as power converters. In some applications, power supplies are used to provide particular voltage characteristics to the load. Switch mode power supplies represent a particular class of power supplies that use switching technology to perform power conversion. Switch mode power supplies have become recognized in the art for their relatively high efficiency.
The performance of a switch mode power supply may be improved by controlling the switching characteristics of the associated power supply circuit. Prior art techniques for controlling switch mode power supplies involve the use of an output voltage control loop. Using such techniques, the output voltage (i.e. the voltage at the load) is measured and compared to a desired output voltage to obtain control information. The control information is used to alter the power supply switching characteristics in a manner that makes the actual output voltage track a desired output voltage.
More recently, it has become possible to control switch mode power supplies by introducing an output current control loop. The output current control loop measures the output current and compares the output current to a desired output current to obtain control information. The control information is used to alter the power supply switching characteristics in a manner that makes the actual output current track a desired output current.
In some prior art power supplies, an output current control loop is combined with an output voltage control loop. The voltage control loop selects a desired output current that will achieve a desired output voltage. The output current control loop, which typically operates more quickly than the voltage control loop, obtains its desired output current from the voltage control loop. In this manner, the output current control loop selects the power supply switching characteristics that will make the actual output current track the desired output current and the actual output voltage track the desired output voltage.
Current control for switch mode power supplies has been disclosed in a number of prior art references, which include:                U.S. Pat. No. 6,314,005 (Nishi et al.);        U.S. Pat. No. 4,678,984 (Henze);        U.S. Pat. No. 6,020,729 (Stratakos et al.);        U.S. Pat. No. 6,031,361 (Burstein et al.);        U.S. application Publication No. 2001/0038277 (Burstein et al.); and        PCT International Application No. WO 03/003554 (Mobers).        
Some prior art switch mode power supplies employ analog current control. While analog current control may be effective at high switching frequencies, analog current control has a number of disadvantages. For example, analog current controllers have limited flexibility. If the application changes, then the components of the analog control system must also change. Typically, a different analog controller is required for each different power supply circuit, whereas a digital controller embodying the same hardware could be programmed differently for different power supply circuits. With regard to testing, analog controllers must typically be assembled into their respective power supply circuits prior to testing, whereas digital controllers may be tested prior to assembly using digital techniques. The components of analog current controllers (e.g. resistors and capacitors) may also degrade over time, causing a corresponding effect on circuit performance.
There is a general need for methods and apparatus for implementing digital current control of various switch mode power supply topologies which ameliorates at least some of the disadvantages of the prior art.